This invention relates generally to signal processing techniques for fiber optic sensor systems. This invention relates particularly to demodulation of signals output from an array of fiber optic interferometric sensors for determining changes in a physical parameter measured by the individual sensors. Still more particularly, this invention is directed to determining the phase offsets in a numerically controlled oscillator for the fundamental carrier and carrier first harmonic in order to eliminate the sign uncertainty in a coherent phase generated carrier demodulator.
Mismatched fiber optic interferometers are commonly used as sensing elements in fiber optic sensor arrays for measuring changes in a parameter such as fluid pressure, acceleration, magnetic field intensity, etc. Such sensing elements measure the time-varying phase delay between optical signals that have propagated along separate optical paths having unequal path length.
Mixing between a reference signal and a data signal is often necessary to extract information from an optical carrier. In interferometric sensing the mixing is typically between a reference signal and a signal whose phase has been modified, or modulated by the parameter being measured.
Modulation is commonly used to transmit information from an information source, such as a sensor system where information is detected, to an information destination, such as a receiver system where detected signals are received and processed. According to conventional modulation techniques, a signal of interest detected by a sensor modulates a carrier signal by modifying one or more characteristics of the carrier signal, such as amplitude, frequency or phase, to form a modulated carrier signal. The modulated carrier signal is then more easily transmitted over the appropriate communication channels to the destination or receiver system where the modulated carrier signal is demodulated to recover the signal of interest.
The fiber optic sensors detect or sense signals that modulate the output phase of the sensor system or interferometer. The modulated carrier can then be transmitted to a receiver system and photodetected. In a system having an array of sensors, the signals are often multiplexed, for example, using time division multiplexing (TDM) and/or wavelength division multiplexing (WDM), as well as frequency division multiplexing (FDM).
Fiber optic sensor systems acquire in the demodulation process one term proportional to the sine of the sensor phase shift and another term proportional to the cosine of the sensor phase shift. The sine of the sensor phase shift is referred to as the quadrature term, Q; and the cosine of the sensor phase shift is referred to as the in-phase term, I. The angle of the phase shift is determined by calculating the ratio I/Q, which is the arctangent of the sensor phase shift. The amplitudes of the sine and cosine terms must be set equal by a normalization procedure to ensure accurate implementation of an arctangent routine to determine the sensor phase shift.
One type of modulation technique used in interferometers and other sensing systems involves the use of phase generated carriers. The time varying phase signal (signal of interest) of each sensor modulates the phase generated carriers to form modulated carriers. Both the signal of interest and the phase generated carriers can be mathematically represented as a Bessel series of harmonically related terms. During modulation, the Bessel series of the signals of interest modulates the Bessel series of the phase generated carriers. The number of terms in the Bessel series of the resulting modulated carriers will be dependent upon the amplitude of the measured or detected signals of interest. The harmonically related terms in the Bessel series of the modulated carriers represent both the measured or detected signals of interest and the carrier signals.
Typical fiber optic sensor systems using phase generated carriers to transmit a detected or measured signal (signal of interest) to a receiver system have used a pair of quadrature carriers with frequencies of either xcfx89c and 2xcfx89c or 2xcfx89c and 3xcfx89c, where xcfx89c is the phase generated carrier frequency. In multiplexed sensor systems, the sensor sampling frequency fs must be selected to ensure that frequencies greater than fs/2 are not aliased into the band of interest below fs/2.
In some systems the optical signal input to the interferometer is a phase generated carrier produced by producing time-dependent variations in the frequency of the optical signal output by a laser. A phase generated carrier may be produced by various techniques. One such technique involves routing the source output through a phase modulator and applying a sequence of separate and different linear ramp voltages to the linear phase modulator to produce step changes in the optical frequency.
In some systems the optical signal input to the interferometer is a phase generated carrier produced by generating time-dependent variations in the frequency of the optical signal output by a laser. A phase generated carrier may be produced by various techniques. One technique involves routing the laser source output through an external phase modulator and applying a sequence of separate and unique linear ramp voltages to the linear phase modulator to produce step changes in the optical frequency.
Another technique for producing a phase generated carrier uses sinusoidal phase modulation of the source signal. Instead of sampling signals associated with separate optical frequencies, the sampling of signals is associated with integration over portions of a period of the phase generated carrier.
Still another technique for producing a phase generated carrier involves the use of a Direct Digital Synthesizer (DDS) containing a numerically controlled oscillator (NCO). In particular, carriers that are 180xc2x0 out of phase with the NCO phase will produce sensor responses with opposite sign after demodulation different than those produced by carriers that are in phase with the NCO phase in the DDS. When coherently combined, sensor responses with opposite signs will combine destructively, which results in an attenuation of the combined output and a reduction in overall system dynamic range.
The present invention significantly increases the dynamic range of the coherent phase generated carrier demodulator by reducing signal attenuation that is caused when individual sensor responses of opposite sign in a synchronous environment are coherently combined.
Apparatus according to the invention for reducing sign uncertainty in a coherent phase generated carrier demodulator in an interferometric acoustic sensor system, comprises an optical signal source that provides a phase generated carrier signal to the acoustic sensor system so that the multi-channel acoustic sensor system produces an acoustic signal that is superimposed on the phase generated carrier signal, the interferometric acoustic sensor system being arranged to provide an optical signal output that includes the phase generated carrier signal and the acoustic signal. The invention further comprises a photodetector arranged to receive the optical signal output from the interferometric acoustic sensor system and a downconverter connected to the photodetector. The downconverter is arranged to separate an in-phase component I and a quadrature component Q of the acoustic signal from the phase generated carrier signal. The invention also includes a coordinate transformer connected to the downconverter. The coordinate transformer is arranged to function as a rectangular to polar converter and provide signals indicative of a polar phase angle between the in-phase component I and the quadrature phase component Q of the acoustic signal.
A method according to the invention for eliminating sign uncertainty in a coherent phase generated carrier demodulator in a multi-channel sensor system comprises the steps of arranging an optical signal source to provide a phase generated carrier signal to the multi-channel acoustic sensor system so that the multi-channel acoustic sensor system produces in each channel an acoustic signal that is superimposed on the phase generated carrier signal and connecting an array of downconverters to the photodetector array. The method further comprises the steps of arranging the array of downconverters to separate an in-phase component I and a quadrature component Q of the acoustic signal from the phase generated carrier signal in each channel and connecting a coordinate transformer to the array of downconverters. The method also includes the step of arranging the coordinate transformer to function as a rectangular to polar converter and provide signals that indicate a polar phase angle between the in-phase component I and the quadrature phase component for each channel.
The invention preferably further comprises the steps of adjusting a phase register in each downconverter in a predetermined number of phase intervals starting at 0xc2x0 and ending at 180xc2x0 and sampling signals output from each channel of the sensor system a predetermined number of times for each phase interval. The method also preferably includes the steps of saving maximum values of the in-phase component I and the quadrature phase component Q, saving phase values that correspond to the maximum values of the in-phase component I and the quadrature phase component Q, and setting each downconverter to the phase value that produced the stored maximum values of the in-phase component I and the quadrature phase component Q for the corresponding channel.
The invention preferably also further comprises the steps of calculating the difference of successive squares of Q to determine a quantity DQ=(Qi)2xe2x88x92(Qixe2x88x921)2, calculating the difference of successive squares of Q to determine a quantity DI=(Ii)2xe2x88x92(Iixe2x88x921)2, summing the quantities DQ and DI to determine a sum term DQs and a sum term DIs, calculating a quantity       R    =                  (                              D            Qs                                D            IS                          )            0.5        ,
adjusting the phase generated carrier gain by an amount proportional to 1xe2x88x92R; and repeating the preceding steps until Rxe2x89xa60.1.
The invention preferably further comprises the steps of sampling signals output from each channel of the sensor array a predetermined number of times, saving the maximum values of Q and I, calculating the ratio Rt of the maximum value of Q to the maximum value of I, and adjusting the signals Q and I if the ratio of their maximum values differs from unity.
An appreciation of the objectives of the present invention and a more complete understanding of its structure and method of operation may be had by studying the following description of the preferred embodiment and by referring to the accompanying drawings.